1. Field of the Invention
The present invention relates to a device and a method for detecting deposition formations on sensor surfaces, which are caused by lubricants. In particular, the present invention relates to such a device and such a method that are suited to be utilized in determining the aging condition of lubricants, in particular lubricating oils used in internal combustion engines. In addition thereto, the present invention relates in particular to such a device and such a method that are suitable for eliminating deposits formed on sensor surfaces. The present invention thus is suited in particular for an xe2x80x9cin situxe2x80x9d detection of the aging conditions of lubricating oils so that, on the basis of such a detection, oil changing intervals may be determined.
2. Description of Prior Art
Lubricating oils, in particular lubricating oils in internal combustion engines, are changed as a rule in fixed time intervals. So far, there are no reliable sensor systems available that are capable of reliably determining the condition, i.e. the aging condition, of a lubricating oil while the lubricating oil is within the engine. There is a number of different sensor systems known that detect various parameters in order to determine the aging condition of the oil on the basis of the parameters detected.
For example, the document DE 4131969 C2 reveals a lubricating oil monitoring means in which the parameters pressure, temperature and viscosity and optionally the pH value are detected in situ in the engine. On the basis of the parameters ascertained, an actual condition of the engine oil is determined. This actual condition is compared to a desired condition and, if the desired condition shows a specific deviation with respect to the actual condition, an alarm signal is output indicating that an oil change is necessary. According to said DE 4131969 C2, the pressure is measured by means of a pressure sensor membrane provided with piezoresistive resistors, the temperature is measured by means of platinum resistors, the pH value is measured by means of a MOS transistor having an H+-sensitive gate electrode, while the viscosity is measured from the relative permittivity, ascertained by means of capacitance structures, or alternatively from the attenuation of sound waves generated by piezo oscillators.
The document WO 98/05953 reveals a surface-wave liquid sensor comprising a transmitting transducer and a receiving transducer constituted by respective interdigital transducers. The number of finger pairs of the interdigital transducers as well as the material of which the electrodes are made are selected such that the volume shear wave generated by the transmitting transducer and the surface shear wave generated by the transmitting transducer have different frequencies.
In addition thereto, the document WO 98/37412 reveals a sensor means for detecting material parameters of a liquid medium, in which a surface-wave liquid sensor is used for detecting the viscosity of an adjacent liquid medium, while one of the interdigital transducers of the surface-wave liquid sensor is used furthermore for determining the relative permittivity of the adjacent liquid medium. Moreover, this document teaches to detect as an additional parameter the conductivity by way of the complex electrical impedance of the interdigital capacitor. On the basis of the parameters thus ascertained, there is determined, in consideration of the temperature dependency, an actual condition of the liquid medium which is compared to a desired condition in order to be thus able to determine an aging condition of the liquid medium, in particular of the lubricating oil.
The known oil condition sensors described hereinbefore thus permit the detection of viscosity, relative permittivity and conductivity of oils in situ in the engine. In this regard, the detection of the viscosity permits a more reliable determination of the condition of the oils than is possible using a mere dielectric measurement. However, the viscosity is a measurement quantity which in practical application is presently determined and evaluated mainly in laboratory analyses of engine oils. In such laboratory analyses, there are determined in addition to the viscosity among other things also the solids contents by means of infrared spectroscopy, as well as the acidification of the oil by determination of the TAN (TAN=total acid number). The oil condition is then assessed in the laboratory on the basis of these parameters.
On the engine test bench, i.e. in measuring the oil condition while the latter is within the engine, the use of surface-wave sensors as described hereinbefore has revealed that these, though permitting the in-situ detection of viscosity, yielded less reliable results. In addition thereto, it is to be pointed out that the methods that can be performed in laboratory analyses for reliably determining the oil condition cannot be employed for an examination of the oil within the motor due to the expenditure thereof.
It is the object of the present invention to provide a simple device and a simple method permitting a reliable determination of the aging condition of lubricating oils.
According to a first aspect of the present invention this object is achieved by a device for determining whether a lubricant to be examined causes the formation of a deposit on a sensor, comprising a sensor element for detecting values of at least one physical parameter of the lubricant, said sensor element being arranged in contact with the lubricant. Furthermore, there is provided a potential difference source for applying a potential difference between the sensor element and the lubricant. Moreover, the device according to the invention comprises an evaluator for evaluating at least two values of the physical parameter that are detected at different potential states between sensor element and lubricant, in order to thus determine whether the lubricant causes a deposit formation on the sensor surface.
According to a second aspect of the present invention the above object is achieved by a method of determining whether a lubricant to be examined effects the formation of a deposit on a sensor element for detecting values of at least one physical parameter of the lubricant, said sensor element being arranged in contact with the lubricant, comprising the steps of:
generating different potential states between the sensor element and the lubricant and detecting the value of the physical parameter in at least two different potential states;
determining whether the lubricant causes the formation of a deposit on the sensor element, on the basis of the at least two detected values of the physical parameter.
The present invention is based on the finding that the use of sensor elements for detecting physical parameters of lubricants may be accompanied by a deposit formation on the sensor element, in particular the sensor surface thereof in contact with the lubricant. Such a deposit formation was noted in measurements on the engine test bench by means of a surface-wave sensor, but not in measurements in the laboratory that were carried out using the same lubricant samples, i.e. oil samples. It has been found out now that this deposit formation is caused by a potential difference between engine block/engine oil and sensor element. This deposit formation on the surface of the sensor exposed to the lubricant or oil, however, strongly impairs the measurement of the physical parameters of the oil, for example the viscosity and the relative permittivity of the same.
Such a deposit formation on the sensor surface may be caused by a potential difference inherently present between engine block/engine oil and sensor. However, the deposit formation interferes with the detection of a physical parameter of the oil by a corresponding sensor, since such a deposit falsifies the output signals of the sensor. By application of an electric potential of defined polarity between sensor and lubricant for a short period of time, which is in the range of a few minutes, it is possible now to effect an additional deposit formation. By application of a potential of opposite polarity, it is possible to remove the deposit again. It is to be noted in this regard that it has been found out that this deposit formation and deposit removal occurs in case of aged lubricants only, whereas a deposit formation does not take place with new lubricants. The reason therefor resides in the fact that the deposit is due to substances contained therein, in particular solids such as soot and the like, which are contained increasingly in the lubricant with increasing aging of the same and which have an electric charge, so that a deposit formation is achieved by application of a predetermined polarity between lubricant and sensor element, while the application of an opposite polarity effects the removal of a deposit present on the sensor element.
The present invention takes advantage of the effect described hereinbefore for determining whether a deposit formation is caused by an aged lubricant on a sensor element, by detecting at least two values of a physical parameter of the lubricant by means of a sensor element in the presence of two different potential states between sensor element and lubricant. On the basis of this determination, the aging of the lubricant can then be determined directly since, as elucidated hereinbefore, the deposit formation is dependent on the aging condition of the lubricant or oil under examination. On the other hand, it is possible according to the invention to take advantage of the aforementioned effect for eliminating a deposit formed on a sensor element, i.e. on the sensor surface thereof in contact with the lubricant, and to then carry out a non-falsified detection of physical parameters of the lubricant, for example the viscosity, the relative permittivity or the conductivity thereof.
In other words, it is possible according to the invention by purposeful application of a potential difference between sensor and lubricant or oil, to measure a sum parameter for the solids contents of the lubricant by means of a sensor that is responsive to changes of the physical boundary conditions at the sensor-lubricant interface. The deposit caused by application of the potential difference can be removed again by application of a potential difference of opposite polarity. After the sensor has been freed from the deposit, a sensor capable of measuring viscosities and/or dielectric quantities and being the same sensor as that used for determining the deposit formation, may be utilized for determining the viscosity and/or the dielectric oil properties.
The present invention thus renders possible a reliable determination of the oil condition.
In addition to the parameters ascertained so far for assessing the aging condition of an oil in situ in an engine, for example relative permittivity, conductivity, viscosity and the like, it is possible with the present invention to determine an additional sum parameter permitting a statement to be made on the aging condition of the oil, namely, as pointed out hereinbefore, the parameter whether the lubricating oil causes a deposit formation on a sensor element. This deposit formation may be dependent on various factors occurring with increasing aging of the lubricating oil, in particular the introduction of soot and other electrically charged solids into the lubricant or oil. In contrast thereto, other parameters, for example the relative permittivity and the conductivity, are also influenced in particular by other changes of the lubricant with increasing aging, such as e.g. the introduction of liquid substances, such as water or fuel, and chemical compounds that may cause thermal oxidation or acidification. The more of these alterations of the lubricant may be detected, the more reliable the assessment of the aging state of the lubricant.
In addition to the detection of an additional sum parameter, namely the deposit formation on a sensor surface by a lubricant, the present invention on the other hand permits the purposeful formation and removal of a deposit, respectively, so that the physical parameters of the lubricant, such as e.g. the relative permittivity, the conductivity or the viscosity, can be detected in a state in which the sensor is free from a deposit. Thus, in addition to the detection of an additional sum parameter, the present invention also renders possible a more accurate detection of the afore-mentioned physical parameters.